1. Field of the Invention
The present invention relates to new aqueous two-component coating compositions based on hydroxy- and/or amino-functional water dilutable resins and new water dispersible polyisocyanate hardeners and to their use in coating and sealing compositions.
2. Description of the Prior Art
Because of their outstanding properties, two-component polyurethane coating compositions have acquired great importance in the coatings sector. A disadvantage is that in most cases relatively large amounts of organic solvents are required for their processing. However, in almost all fields of use, high-solids and especially water dilutable coating compositions are required to reduce or avoid solvent emissions to the extend possible.
Until a few years ago, it did not seem possible to use water as a solvent for two-component polyurethane coating compositions because isocyanate groups can react not only with the hydroxyl groups of the resin to give urethanes, but also with water to form urea and carbon dioxide. As a result, the processing time, the application reliability and the ability to obtain adequate blister-free coating thicknesses were made difficult, and the resistance properties of the coating compositions and coatings deteriorated to values which no longer met practical requirements.
In recent years efforts have increasingly been made to reduce these problems. EP-A 358,979 discloses that selected polyhydroxy polyacrylate secondary dispersions can be combined with polyisocyanates having free isocyanate groups to give aqueous two-component compositions.
It is also possible to apply this principle to other hydroxy-functional resin dispersions, which means that the properties of the coating compositions can be varied. For example, EP-A 557,844 describes two-component polyurethane coatings based on hydroxy-functional primary dispersions, EP-A 543,228 describes those based on polyester-polyacrylate hybrid dispersions, EP-A 741,176 describes those based on alkyd resins emulsified with foreign components, EP-A 496,205 describes those based on urethane-modified polyester dispersions, and EP 542,105 describes those based on mixtures of various types of resin.
Both hydrophobic and hydrophilic, self-emulsifying polyisocyanates can be used as the polyisocyanate component in the aqueous two-component polyurethane systems. Hydrophilically modified polyisocyanates, such as those described in EP-A 206,059, in EP-A 540,985 and U.S. Pat. No. 5,200,489, are particularly suitable crosslinking agents. Such polyisocyanate mixtures containing polyether-urethanes can be emulsified very easily, e.g., by simple manual stirring, into the aqueous resin dispersion, which is of substantial importance, in particular for automotive refinish car repair and original equipment manufacture. Due to the simplified incorporation of the hardener, a more homogeneous thorough mixing of the components is achieved, and the application reliability and optical properties, especially the gloss of the coatings, are increased significantly.
However, when these polyether-modified polyisocyanates are used, because of the additional hydrophilic groups introduced into the system and the reduced NCO functionality of the crosslinking agent due to the hydrophilic modification, other important properties of the coating composition, in particular the resistance to solvents and water, are impaired. Also, the compatibility of certain dispersions, e.g. primary dispersions (emulsion copolymers), even with the known hydrophilic polyisocyanates, is often not yet adequate, so that cloudy clear varnish films are often obtained. The problems of a limited pot life and the occurrence of reaction blisters in high coating thicknesses also have not yet been satisfactorily solved.
An object of the present invention is to provide new aqueous two-component polyurethane coating compositions with improved resistance to solvents and water combined with a high gloss, reduced haze, prolonged pot life, high application reliability and adequate blister-free coating thicknesses. These new coating compositions should be suitable in particular for automotive refinish and original equipment manufacture, both as a priming filler, filler, top coat and clearcoat.
Surprisingly, it has been possible to achieve this object with the coating compositions according to the invention, which are based on hydroxy- and/or amino-functional water dilutable resins and new water dispersible polyisocyanate hardeners.
The present invention is based on the surprising observation that by reacting low-monomer polyisocyanates with monofunctional polyethylene oxide polyether alcohols to form allophanate groups, it is possible to prepare new water dispersible polyisocyanate mixtures of high isocyanate functionality which can readily be stirred into aqueous compositions considerably more easily and in a more finely divided form at significantly lower polyether contents than prior art water dispersible polyisocyanates which are prepared from the same polyether alcohols with the exception that the polyether chains are connected to the polyisocyanate via urethane groups. Because of their outstanding emulsifiability in water at lower polyether contents in combination with a higher isocyanate functionality, the use of these new polyisocyanate mixtures as the hardener component in aqueous polyurethane coating compositions results in coatings with outstanding optical properties, in particular high surface gloss, flow and high transparency, and also significantly improved water and solvent resistance.